Pulse train, nuclear magnetic resonance tomograph and imaging method

ABSTRACT

The invention relates to a pulse train, to a nuclear magnetic resonance scanner, which comprises means for generating this pulse train, and to an imaging method in which the pulse train according to the invention is used. The pulse train according to the invention comprises an α high-frequency pulse, a preceding 180° pulse or a preceding 180° and a 90° pulse that precedes the 180° pulse, as well as a slice selection and a k-space line coding as well as an acquisition module, which is subsequent thereto for purposes of generating data. This invention is characterized in that the acquisition module for generating data results from at least two slices. By means of the pulse train and imaging method according to the invention, 20 slices of an image can be acquired at 16 different points in time for a matrix size measuring 256×256 during a measuring time of 8 minutes and 28 seconds. As a result, T 1  relaxation times can be used for the first time for imaging methods in medical diagnosis.

This disclosure relates to: a pulse train comprising at least an ahigh-frequency pulse, a preceding 180° pulse, or a preceding 180° and a90° pulse that precedes the 180° pulse, a slice selection and a k-spaceline coding and an acquisition module subsequent thereto; a nuclearmagnetic resonance scanner; and an imaging method.

Already at the time when the method of magnetic resonance imaging (MRI)was invented, it was anticipated that this method would allow more thanjust simple, qualitative imaging and that it would, in fact, be suitablefor generating quantitative imaging. On the one hand, MRI is a maturemethod that is used on a daily basis in clinical imaging for simple,qualitative image depictions. On the other hand, MRI is a very importantinstrument for science and industry in a wide array of application areassuch as quality control, pre-clinical evaluation of drugs in thepharmaceutical industry as well as in the determination of the pore sizein rock samples in the petrochemical industry. Quantitative imaging isneeded for rock samples and this is also done. The MRI signals arerendered more sensitive or weighted by means of appropriate andcontrolled manipulation of the appertaining parameters such as pulsetrains, in order to show the influence of selected parameters. Generallyspeaking, when a series of differently weighted images are acquired andwhen suitable models are employed, it is possible to generatequantitative representations of the selected parameters. In this manner,quantitative images of samples can be created for purposes ofdetermining a certain parameter such as diffusion, proton density orspin-lattice relaxation time.

The term “sample” in the case at hand here should be construed in itsbroadest sense and it encompasses both living and non-living material.

Various methods are known in which a sample is examined by means of anexcitation pulse and several rephasing pulses.

In the method of this type, the sample is excited by means ofelectromagnetic radiation at an energy level that is suitable for theexcitation.

It is known procedure in nuclear magnetic resonance tomography to obtaininformation about a sample by exciting echo signals of the sample.

In nuclear magnetic resonance tomography, atom nuclei possessing amagnetic moment are aligned by applying an external magnetic field, aprocess in which the nuclei execute a precessional motion having acharacteristic circular frequency (Larmor frequency) around thedirection of the magnetic field. The Larmor frequency is a function ofthe strength of the magnetic field and of the magnetic properties of thesubstance, especially of the gyromagnetic constant γ of the nucleus. Thegyromagnetic constant γ is a parameter that is characteristic for eachtype of atom. The atom nuclei have a magnetic moment μ=γ×p wherein pstands for the spin of the nucleus.

A substance to be examined or a person to be examined is exposed to auniform magnetic field during nuclear magnetic resonance tomography. Theuniform magnetic field is also referred to as the polarization field B₀,and the axis of the uniform magnetic field as the z-axis. The individualmagnetic moments of the spin in the tissue precede with theircharacteristic Larmor frequency around the axis of the uniform magneticfield.

A net magnetization M_(z) is generated in the direction of thepolarization field, whereby the randomly oriented magnetic componentscancel each other out in the plane perpendicular thereto (x-y plane).After the uniform magnetic field has been applied, an excitation fieldB₁ is additionally generated. The excitation field B₁ is polarized inthe x-y plane and displays a frequency that is as close as possible tothe Larmor frequency. As a result, the net magnetic moment M_(z) can betilted into the x-y plane, so that a transverse magnetization M_(t) iscreated. The transverse component of the magnetization rotates in thex-y plane with the Larmor frequency.

Through a variation of the excitation field over the course of time,differing time sequences can be generated for the transversemagnetization. Various slice profiles can be realized in conjunctionwith at least one applied gradient field.

NMR imaging methods select slices or volumes that yield a measuringsignal under the appropriate emission of high-frequency pulses and underthe application of magnetic gradient fields; this measuring signal isdigitized and stored as a one-dimensional or multi-dimensional field ina measuring computer.

This multidimensional field resulting from the measurement can bedepicted in a spatial frequency space, the k-space. The coordinates ofthis spatial frequency space result from k=−γ∫Gdt. The outer area of thek-space defines the structures of the reconstructed image while theinner area defines the contrast.

The desired image information is then acquired (reconstructed) from thegathered raw data by means of one-dimensional or multi-dimensionalFourier transformation. Before that, there could be a need for themeasured data of the multidimensional data field to be arranged in thedata memory in such a way that the appertaining k-spaces yield thecorresponding slices. Sorting procedures are implemented for thispurpose.

A reconstructed slice image consists of pixels, and a volume data recordconsists of voxels. A pixel (picture element) is a two-dimensional imageelement, for instance, a square. The image is made up of pixels. A voxel(volume pixel) is a three-dimensional volume element, for instance, aright parallelepiped. The dimensions of a pixel are in the order ofmagnitude of 1 mm², and those of a voxel are in the order of magnitudeof 1 mm³. The geometries and extensions can vary.

Seeing that, for experimental reasons, it is never possible to assume astrictly two-dimensional plane in the case of slice images, the termvoxel is often employed here as well, indicating that the image planeshave a certain thickness.

Little attention has been paid to the representation of the spin-latticerelaxation time, T₁, since most of the methods presented in theliterature require long acquisition times that render these methodsunusable for routine clinical examinations.

The advantage of the rapid data acquisition that was attained employingthe rapid “Inversion-Recovery (Inversion—Relaxation) EPI (echo-planarimaging) Method” by R. J. Ordidge et al. in Magnetic Resonance inMedicine 16, 238-245 (1990) did not become well established because EPIis not a method in widespread use. In fact, inherent artifactsassociated with this method have prevented the utilization of thisotherwise elegant method. This is particularly true in those cases whereimaging of the highest quality is needed, such as in the segmentation ofthe human brain. Other quantitative imaging methods (Deichmann et al. inJournal of Magnetic Resonance, 96, 608-612 (1992); Blüml et al., MRM 30,289-295 (1993); Deichmann et al. in Magnetic Resonance in Medicine, 42:206-209 (1999)) are slower than IR-EPI and are not fast enough to attainpractical significance. The two approaches are based mainly on thespectroscopic Look-Locker methods Look DC and Locker DR (The Review ofScientific Instruments, volume 41, no. 2, 250-251, (1970)), which makesuse of successive excitation pulses during a longitudinal relaxation soas to gather numerous points in time during the relaxation. A moreeffective time-representation scheme that neutralizes movement artifactscan be created in this manner.

The original “snapshot FLASH” method by Deichmann et al. in Journal ofMagnetic Resonance, 96, 608-612 (1992) calls for long acquisition timessince the initial magnetization has to be completely re-established. Inthe case of a high spatial resolution, the time resolution is markedlyrestricted. This especially has an effect in nuclear magnetic resonancescanners without a high-performance gradient system.

In nuclear magnetic resonance tomography of the brain, especially of thehuman brain, there is need to acquire measuring points over the entirebrain volume, thus leading to the highest possible resolution of a sliceimage while generating as many slice images as possible within a briefperiod of time. This requirement is particularly pressing when sick orseverely injured people have to be diagnosed quickly. The prior-artmethods need a very long acquisition time of more than one hour and arethus not suitable for clinical use. The methods based on “snapshotFLASH” approach are faster. However, since these are single-slicemethods, a high spatial resolution cannot be achieved. Methods based onEPI are very fast but they entail numerous disadvantages which lie inthe very nature of the methods. These artifacts include ghost imagesresulting from phase errors and at times severe geometricalinterferences.

SUMMARY OF THE INVENTION

Therefore, the invention has the objective of creating an imagingmethod, a nuclear magnetic resonance scanner and a pulse train which,within the shortest time possible, generate a slice image or a sequenceof slice images of the brain at an extremely high resolution.

This objective is achieved according to embodiments of the invention atleast in part by an acquisition module which generates data results fromat least two points in time, and at least two slices.

The imaging method according to an embodiment of the invention allows,for example, the acquisition of 20 slices at 16 different points in timefor a matrix measuring 256×256 within a measuring time of 8 minutes and28 seconds.

Advantageous refinements of the invention are given in the subordinateclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures show various test results and parameters.

The following is shown:

FIG. 1—a scanning scheme of the pulse train according to the invention;

FIGS. 2a, 2 b—a pulse train according to invention;

FIG. 3—representation of the echoes obtained in the k-space;

FIG. 4—reconstructed relaxation map of a ghost image;

FIG. 5—quantitative in vivo T₁ maps of a cross section through the humanbrain.

DETAILED DESCRIPTION

The invention will be explained below by way of an example.

In the execution of the method according to the invention for thedetermination of the longitudinal relaxation constant T₁, the sample,for instance, the skull of the patient to be examined, is placed intothe coil of a nuclear magnetic resonance scanner. The sample lies in thecenter of the homogeneous, static basic magnetic field B₀. The spatialresolution is done using magnetic fields that change over the course oftime (gradients). Due to their design, these are arranged in a Cartesiancoordinate system. In order to obtain data, a high-frequency excitationfield has to be generated. The excitation frequency is dependent, on theone hand, on the static basic field and, on the other hand, on the typeof nucleus of the sample to be examined. Once excitation has beencompleted with a non-slice-selective (or else volume-selective) 180°pulse or, as an alternative, a 90° pulse followed by a 180° pulse—bothof which are non-slice-selective or both of which arevolume-selective—the data is acquired in two sequences that differ fromeach other. After the 180° pulse, the transversal magnetization M_(xy)has to be zero (for instance, due to gradient spoiling). In both cases,for purposes of acquiring data, an high-frequency pulse (α) is applied,combined with gradient sequences in order to generate at least onek-space-coded echo. This measuring scheme (high-frequency pulses (α) andgradient sequence) will be hereinafter referred to as the acquisitionmodule. After the measurement has been completed, the k-space has beencompletely scanned. The high-frequency pulse (α) serves to excite aselected slice or slice thickness and can assume the pulse forms thatare commonly known to the person skilled in the art. These are, forinstance, Gauss pulses or sinc pulses. The pulse trains for bothpossibilities can be seen in FIGS. 2a and 2 b. The square brackets shownin FIGS. 2a, 2 b schematically depict the use of the sequence presentinside the square brackets (acquisition module) on slices or points intime. In this context, FIG. 2b relates to the acquisition scheme inwhich three k-space lines are measured after each high-frequency pulse(α). FIG. 2 describes the acquisition case for one k-space line perhigh-frequency pulse (α). Parallel to these high-frequency pulses, theslice of interest is determined on the basis of the slice selectiongradient G_(z) (G_(s)). Subsequently, the voxel magnetizations arerephased by means of a slice selection gradient having the oppositealgebraic sign. During the rephasing, a gradient G_(x) (G_(r)) isapplied that dephases the voxel magnetizations, thus effectuating acoding in the k-space. At the same time, a gradient G_(y) (G_(p)) isapplied in order to dephase the voxel magnetization so as to, in turn,carry out a k-space coding. The dephasing in the x-direction, in turn,is compensated for by means of a downstream G_(x) gradient having theopposite algebraic sign. The data acquisition takes place at the sametime as this G_(x) gradient with its rephasing effect. The surface areasbelow the gradients are determined in such a way that the gradient echoappears in the middle of the acquisition window (FIG. 2a). But otheracquisition methods are also conceivable such as, for example,asymmetric gradient echo. Thus, one line of the k-space has beenmeasured. The above-mentioned steps are repeated as many times as thereare slices or points in time to be measured. Subsequently, the inversionpulse (180° pulse) or the combination of 90° pulse and inversion pulseis applied once again. This procedure is repeated, but in each case, thenext k-space line is measured for all slices. The number of repetitionsresults from the number of phase coding steps. FIG. 1 shows thefundamental relaxation curve of the longitudinal magnetization and thedata blocks that are acquired according to the invention in thisprocess. Inside the data blocks—points in time—the k-space line(s) forthe n slices are measured. The measurement is completed once thek-spaces for the slices or points in time have been completely scanned.

In this manner, the time span for a complete longitudinal relaxation isused according to the invention as efficiently as possible for purposesof acquiring data about several slices and several points in time. Thisway, more data can be acquired over a given period of time. The termdata as employed by the invention refers to the gathering of informationfrom several slices and several different points in time.

In order to configure the data acquisition more efficiently, followingthe slice-selective excitation by the high-frequency pulse (α), severalk-space lines can be successively measured for each slice. This processis also referred to as segmentation. The variant of the method presentedhere describes the logical limit of segmentation since only one k-spaceline is acquired per segment. The data acquisition can contain severalsegments such as, for instance, 5 or 7. In the method being describedhere, 3 k-space lines—one line per segment in each case—are acquiredsuccessively. This is done by reversing the algebraic sign of thegradient G_(x) after the data acquisition of the preceding k-space line.Prior to that, the corresponding k-space line is coded by the phasecoding gradient G_(y) (FIG. 3).

In summary, the four pulse trains presented here can be described bymeans of the following sequence scheme:

(a) 180°−[(α gradient echo measurement of one k-spaceline)×slices×points in time]

(b) 180°−[(α gradient echo measurement of several k-spacelines)×slices×points in time]

(c) 90°/180°−[(α gradient echo measurement of one k-spaceline)×slices×points in time]

(c) 90°/180°−[(α gradient echo measurement of several k-spacelines)×slices×points in time]

In order to be able to use a Fourier transformation after the dataacquisition, the previously acquired data has to be sorted. This is donein such a way that complete data records (k-spaces) are obtained eachtime for the appertaining slices at various points in time. This sortingtakes place prior to the actual reconstruction once the measurement hasbeen completed. The sorting process for the sequence types, based on thescheme of the measurement of one k-space line per high-frequency pulse(α), differs from that of the measurement of several k-space lines perhigh-frequency pulse (α). For the sequences that only acquire onek-space line per high-frequency pulse (α), only the chronological datasequence determined by the pulse train has to be sorted in the measuringmemory according to the k-space definition. With the sequence types thatmeasure several k-space lines per high-frequency pulse (α), it isnecessary to additionally sort the data in such a way that each line ofa k-space has the same algebraic sign.

FIG. 4 shows the reconstructed relaxation map of a ghost image. Nineindividual tubes are found in the ghost image. Eight of these tubes arefilled with Gd-DTPA at differing concentrations, which can be clearlyseen in the different grayscale values. The measurement was performedemploying a method according to the invention in which after each 180°pulse, one k-space line per high-frequency pulse (α) is recorded.

The imaging method according to the invention, the nuclear magneticresonance scanner as well as the pulse train make it possible, forexample, to acquire 20 slices at 16 different points in time for amatrix measuring 256×256 within a measuring time of 8 minutes and 28seconds for purposes of determining the longitudinal relaxation time.Naturally, the number of slices, the matrix size as well as the numberof measured points in time can all vary. The accuracy of the methoddescribed here in comparison to the standardized spectroscopic method ishigher than 95%, in addition to which it entails the considerableadvantage that data records of several slices and points in time caneven be measured at all, or measured within a much shorter measuringinterval.

The nuclear magnetic resonance scanner according to the invention isequipped with means that allow the generation of the pulse trainaccording to the invention. The term “means that allow the generation ofthe pulse train according to the invention” refers to a data carrierthat stores the information needed to emit the pulse train according tothe invention and that emits said data to a high-frequency unit forpurposes of providing the high-frequency pulses. In order to generatethe pulse train according to the invention, coils that generate themagnetic fields that can be varied over time (gradients) as well as acomputer that controls all system-related components are employed.Moreover, the nuclear magnetic resonance scanner according to theinvention is fitted with an electronic evaluation system with which themeasured data is sorted according to the invention.

EXAMPLE

Ghost image measurement with 90°/180° three k-space line sequence. T_(R)(=repetition time)=13 msec; TI (=inversion time)=30 msec; TD (=delaytime)=3 sec; α (=flip angle)=6°; 4 slices; slice thickness=8 mm; matrixsize=256; FOV (=representation area)=250 mm; 48 points in time.

What is claimed is:
 1. A group pulse-sequence signals suitable fordetermining spin-lattice (T1) relaxation times in a magnetic resonancescanning process, the group comprising: a high-frequency pulse; at leasta 180° inversion pulse preceding the high frequency pulse; pluralslice-selection gradient sequences; a k-space line coding gradient;wherein each of the pulse-sequence signals group are arranged in timesuch that data collection from two or more points in time and from atleast two spatial slices is enabled to permit use of T1 relaxation timesin the magnetic resonance scanning process.
 2. The group ofpulse-sequence signals of claim 1, further comprising a preceding 180°and a 90° pulse that precedes the 180° inversion pulse.
 3. The group ofpulse-sequence signals of claim 1, further comprising at least twoconsecutive gradients G(r), the at least two consecutive gradients G(r)being applied in a read-out direction and having reversed algebraicsigns with respect to each other.
 4. The group of pulse-sequence signalsof claim 3, wherein the at least two consecutive gradients G(r) comprisethree or more gradients applied in a read-out direction and havingreversed algebraic signs with respect to each other.
 5. A magneticresonance imaging method which uses spin-lattice (T1) relaxation timesfor medical diagnosis, the method comprising: generating the group ofpulse-sequence signals of claim 3; emitting high-frequency pulses andapplying two or more magnetic gradient fields in response to thegenerated group of pulse-sequence signals; selecting two or more areasin which a nuclear magnetic resonance occurs; detecting respectivemeasuring signals representative of the nuclear magnetic resonance ineach of the selected areas; and reading out data during application ofthe at least two consecutive gradients G(r).
 6. A magnetic resonanceimaging method which uses spin-lattice (T1) relaxation times for medicaldiagnosis, the method comprising: generating the group of pulse-sequencesignals of claim 1; emitting high-frequency pulses and applying at leastone magnetic gradient field in response to the generated group ofpulse-sequence signals; selecting at least one area in which a nuclearmagnetic resonance occurs; and detecting at least one measuring signalrepresentative of the nuclear magnetic resonance in the selected area.7. The magnetic resonance imaging method of claim 6, further comprisingsorting data representing the measured signal; and applying a Fouriertransformation to the sorted data.
 8. A nuclear magnetic resonancescanner suitable for characterizing spin-lattice (T1) relaxation times,the scanner comprising: means for generating the group of pulse-sequencesignals of claim 1; plural magnetic field generators responsive to oneor more of the generated pulse sequence signals; and evaluation meansfor at least sorting data measured during one or more data collectionperiods.
 9. The nuclear magnetic resonance of claim 8, wherein theevaluation means comprises means for applying a Fourier transformationto the sorted data.
 10. A magnetic resonance imaging method which usesspin-lattice (T1) relaxation times for medical diagnosis, the methodcomprising: generating pulse-sequence signals comprising ahigh-frequency pulse, at least a 180° inversion pulse-preceding thehigh-frequency pulse, plural slice-selection gradient sequences, ak-space line coding gradient, and at least two consecutive gradientsG(r) applied in a read-out direction and having reversed algebraic signswith respect to each other; emitting high-frequency pulses and applyingat least two magnetic gradient fields in response to the generated groupof pulse-sequence signals; selecting at least two areas in which anuclear magnetic resonance occurs; and detecting at least two signalsrepresentative of the nuclear magnetic resonance in the selected areas;and determining one or more spin-lattice (T1) relaxation times relatingto material present in the selected areas.
 11. A nuclear magneticresonance scanner suitable for characterizing spin-lattice (T1)relaxation times, the scanner comprising: means for generatingpulse-sequence signals comprising a high-frequency pulse, at least a180° inversion pulse preceding the high frequency pulse, pluralslice-selection gradient sequences, a k-space line coding gradient, andat least two consecutive gradients G(r) are applied in a read-outdirection which have reversed algebraic signs with respect to eachother; means for providing plural magnetic fields in response to one ormore of the generated pulse-sequence signals, wherein at least twoconsecutive gradients G(r) are applied in a read-out direction whichhave reversed algebraic signs with respect to each other; and evaluationmeans for at least sorting data measured during one or more datacollection periods and for performing a Fourier Transformation on themeasured data.